Inductive chargers for mobile telephones are already known. The publication GBA-A-2 291 291 for example concerns a contactless charger with a primary winding which transmits charging power without making direct electrical contact with a secondary winding in a mobile telephone, for charging batteries via a charging rectifier. The primary winding is excited by a power oscillator. In this device the tractive force of the magnetic coupling field apparently produces a problem when the mobile telephone is disconnected from the charger during charging in order to receive calls. To switch off the magnetic coupling field during usage, when a call arrives or when a predetermined key is activated on its keyboard, the mobile telephone sends an IR signal to the charger to stop the power oscillator. This can also be triggered by a mechanical switch located in the charger's housing. The switch can also manually switch the charger on and off An expansion of the solution provides for activating this mechanical switch with the weight of the mobile telephone itself, and to turn the device on by hanging up the mobile telephone. But an automatic switch-off of the power supply when the batteries are sufficiently charged is not provided. Beyond that the charger is also unable to detect whether a mobile telephone has actually turned the device on.
To provide high mobility and continuous utilization of the device, in a mobile telephone, for example, the batteries have a high capacity and must be reenergized in the shortest time by network power or by the on-board power of a vehicle. This requires an extremely light and compact charger that supplies a relatively large amount of power to the mobile device during short charging times, for so-called rapid charges. Due to the compactness of mobile telephones, only small size inductive couplers can be used, which supply the power to the batteries with little loss and a minimum of control effort. In the interest of reliability and ease, the charger and the corresponding mobile device must interact as much as possible as an automatic system. This means the charger must always automatically switch to a stand-by mode when the mobile device is too far away or the batteries are already charged. In the stand-by mode the charger must be able to remain connected to a power source such as the local network or the on-board network of a motor vehicle in an unlimited manner and without creating safety and reliability problems. In that case the device must use a minimum of power and only emit a low magnetic output. This prevents any metallic objects in the vicinity, or a device which is mechanically connected without an inductive coupler or with a charged battery, to heat up to an undesirable degree due to magnetic induction. The charger can only switch on and continuously emit the alternating magnetic field if a device with a corresponding inductive coupler uses actual power to load the alternating magnetic field.
An inductive charger that fulfills these requirements was already described in the old German patent application DE-197 41 279. It stated that a resonance transducer with resonating circuits on both the primary and the secondary side is particularly well suited for an inductive transmission. The resonance transducer is designed as a push-pull power oscillator with a positive capacitive feedback; it can operate with little switching effort under different loads and is distinguished by low interference emissions in the high-frequency range and low power loss. Switching frequencies above 500 kHz can also be obtained without problems, which allows using low volume and low weight U-shaped ferrite cores in the inductive coupler.
Each push-pull branch of the oscillator contains a separate resonance circuit with an inductance that is formed by a primary winding of the coupler. Although both primary windings are located on a common core, they are physically separated from each other so that each one produces an alternating magnetic field in a different space which can be separately influenced. Because of the separate primary windings, the resonating circuits of the push-pull branches have low magnetic coupling and therefore react differently when the spaces are subjected to unequal loads.
Since foreign bodies cause unequal loads as a rule, they can be detected in the primary circuit by means of differences in the predetermined current and voltage values between the push-pull branches. For that reason a control circuit in the charger detects both differences and changes in the oscillator's power consumption and thus detects different loads in the secondary part of the alternating magnetic field such as full load, no load and faulty load due to a foreign body, and reduces the power supply or switches it off.
However, practical embodiments of the solution have shown that a precise construction of the coupler windings is difficult due to the low number of windings and the small U-cores. Therefore different voltage and current values occur in spite of equal secondary loads in the push-pull branches, so that the detection proposed in the patent application DE-197 41 279 is unsatisfactory without an adjustment of the symmetry and does not reliably detect a connected device.